The Characteristics Of Hourly Precipitation In Tianshan Mountain During Summer

By using station observed hourly precipitation data,the climatic features of the summer hourly precipitation over and around Tianshan Mountain are investigated.The results show that the daily rainfall amount,precipitation frequency and duration are closely related to the topography.The precipitation amount,frequency and duration over high elevation areas are higher than those in the relatively low elevation areas.The rainfall in Tianshan Mountain is characterized with more weak rainfall hours and less intense rainfall.With the altitude increase from east to west in the Yili Valley,the precipitation especially weak rainfall increases as well.In the south-north direction,the mountain ranges possess the maximum precipitation amount and frequency.The distribution of precipitation intensity is rather even,only the intensity in the Yili Valley and that north to the range of Mid-Tianshan area are slightly higher than other areas.In the Mid-Tianshan,the precipitation over the mountain is much larger than surrounding areas,as well as weak rainfall frequencies.Most precipitation events in Tianshan Mountain are initiated during afternoon and characterized with short duration.Rainfall peaks during late afternoon to evening in Yili Valley,and its precipitation often companions with southwest and northeast wind.While rainfall peaks at different times in the three regions of Mid-Tianshan area.Mountain stations peaks in the afternoon,north in the evening and south in the morning.Their companion wind direction differs as well,mountain with west and north wind,north and south with corresponding downslope wind.The features of the diurnally varying summer precipitation over and around the central Tianshan Mountain are investigated.Both the hourly rainfall data observed at eight stations along a transect across the mountain and the convective index derived from the satellite data show that there are three distinct regimes: the early morning peak at stations to the south of the mountain,the late afternoon peak at stations on the mountain,and the night peak at stations to the north of the mountain.The relation between regimes of diurnal variation is analyzed.Through defining the regional rainfall event(RRE),the initial stations of each RRE are recorded.The early morning rainfall in the southern periphery of the mountain is triggered locally in the southern basin.Both the late afternoon peak over the mountain and the night peak in the northern periphery are contributed by mountain originated rainfall events.These rainfall events appear over the mountain in the afternoon and some of them move northward and lead to the nocturnal rainfall in the northern basin.By using reanalysis data and WRF's simulated results,the diurnally variation features of circulation conditions over and around Tianshan Mountain are analyzed.The triggering of convection in the afternoon over the mountain and that in the early morning in the southern basin is related to the diurnally varying wind and thermodynamic conditions over and around the mountain.Low-level convergence with thermodynamic instability appears at noon over the mountain and at night in the southern basin just before the start of the convection.